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"It is still busy in the emergency department - where care is focused on those who are most acutely and severely unwell.

"Work usually undertaken by striking [registrars] and house officers is being taken up by senior doctors, clinical nurse specialists and those junior doctors who are not union members or have chosen not to strike.

"The cumulative impact of this longer period of strike action is expected to be felt over the latter half of the strike."

He said 14 members of the Resident Doctors' Association, the union behind the strike, were working.

"Some of them feel strongly that doctors shouldn't strike."

Also working were 15 non-union resident doctors, Roberts said.

The association's national secretary, Deborah Powell, said members held a protest today at Hagley Park, part of which is across the road from Christchurch Hospital. No protests were held in Auckland.

At least 3025 association members are eligible to strike.

The strike began at 7am today and ends at 8am on Friday, making it, at 73 hours, significantly longer than the 48-hour strike last October. Eighteen of the 20 DHBs are affected. The West Coast and Taranaki DHBs are not affected.

Thousands of patients have had non-urgent procedures or outpatient appointments postponed to allow hospital staff to focus on emergency and acute care.

In talks with the union, the DHBs have agreed to reduce rosters of 12 days in a row, which were considered a risk to patients and the doctors who worked them, to a maximum of 10.

This results in more days off.

Powell said: "There is a reduction in pay associated with us not working the hours."

Roberts, who was not present at the union-DHBs talks, said he understood Powell proposed a model that, although not fully matching the reduced hours, would go a considerable way towards that.

The Herald understands the proposal would lead to doctors not claiming pay for around one third of the days off.

However, the union also wants members to be able to take the days off around weekends and the DHBs have not agreed to this, arguing it would leave them with too few resident doctors at key times, especially on Mondays and Fridays, and too many on Wednesdays.